This invention relates to microwave transmitting and receiving arrangements and more particularly to such arrangements in which a magnetron is used as a microwave source, as, for example in radar systems in which a common antenna is employed for transmitting signals originating from said magnetron and receiving return signals from a target.
Conventionally such a radar system may use an arrangement as shown schematically and partly in section in FIG. 1. A magnetron 1 generates microwave energy during transmission which is coupled into a waveguide section 2 via a co-axial line 3 terminated in a probe 4 and a ceramic or glass dome 5. The magnetic member 6 of the magnetron 1 is of "horseshoe" form, as viewed, forming a magnetic loop from pole piece to pole piece.
Energy from the magnetron 1 is propagated in both directions along the waveguide section 2. In one direction (to the right as illustrated) the energy is incident on an end wall of the waveguide section 2, which is known as a backstop 7. This acts as a short circuit and is positioned such that the incident energy is reflected and adds to the energy propagated in the opposite direction to give a combined signal.
The combined signal is coupled into a branch duplexer 8 which has an antenna arm 8A and a receiver arm 8B. Part of the signal passes along the antenna arm 8A. The remainder enters the receiver arm 8B where it is coupled into a protection unit 9. The unit 9 comprises a short section of waveguide having therewithin two oppositely poled p.i.n. diodes 10 and 11. Such protection units are sometimes referred to as diode limiters. The unit 9 acts to reflect energy having a high amplitude and transmit low amplitude energy. The energy from the magnetron 1 has a relatively high amplitude and hence is reflected by the protection unit 9. The positioning of the protection unit 9 is such tht the reflected energy is added to the signal in the antenna arm 8A and thus most of the energy from the magnetron 1 is passed, via an antenna port 12 located at the end of the antenna arm 8A, to an antenna (not shown) where it is transmitted.
During the systems receiving mode, a signal received at the antenna is passed via the antenna port 12 to the antenna arm 8A. Part of the signal then enters the receiver arm 8B and part the waveguide section 2. That part in the waveguide section 2 is reflected by the backstop 7 and is directed into the receiver arm 8B where it is added to the rest of the signal. The resultant signal is coupled into the protection unit 9. This resultant signal has a very low amplitude compared with the magnetron output, insufficient in fact to trigger the protection unit 9, and thus passes through to the output port 13 of the protection unit 9 with little attenuation. Output port 13 consitutes a receiver port to which a receiver (not shown) is connected.
There are a number of disadvantages associated with such an arrangement. The apparatus is bulky and comprises a number of sections which must be accurately aligned. The sections must also be carefully designed and manufactured to have the correct dimensions. The apparatus therefore is also quite expensive.
The present invention seeks to reduce or eliminate these disadvantages.